That said, I would travel (and have travelled) on other festivals, but would not on RH/YK. To my own view they are non-negotiable.
-- Mike
If you have never been to Israel during the holidays, you might be interested to see the difference between how Yom Kippur and Rosh Hashana are marked here.
Everything is closed on Yom Kippur. The roads are empty. Even the most non-religious Jew would not think of driving (and most Arabs don't either). Children, on the other hand, enjoy riding their bicycles in the usually-traffic jammed streets of Tel Aviv.
A great percentage of Israelis go to synagogue on Yom Kippur, many others (especially on kibbutzim) have some sort of activity for spiritual reflection, and everybody goes to a family or other communal dinner to break the fast.
Except for the very Orthodox, however, Rosh HaShana is a very different thing. It is marked by trips to the sea or the mountains and those who can afford it often get on planes to Cyprus, Rhodes, Turkey, or other nearby countries.